1. Field of the Invention
The present invention generally relates to a method for detecting a tracking short in an electric circuit. More particularly, the invention relates to distinguishing the normal variation of current caused by the use of several electric devices from the tracking short current.
2. Description of the Relevant Art
A tracking short may occur once a carbonized conductive passage is formed in the isolator positioned between two points of an electric circuit, current may then flow through the formed conductive passage. Then, the carbide in the passage can become over heated causing the carbonization of the isolator around the passage. As this process is repeated, the amount of current flowing between the two points in the electric circuit increases. As the current flow increases, the heat generated can finally cause a fire or burning. Such a phenomenon is called xe2x80x98tracking shortxe2x80x99. Tracking short it likely to occur especially in the outlets or plugs that have not be used for a long time, and once occurring, it is tends to develop into a fire. One conventional method for detecting the occurrence of a short in an electric circuit, is known and includes: detecting a current value in the electric circuit using a current transformer; converting the current value to a voltage value by a current-voltage converter; A/D converting the voltage value by an A/D converter; and judging whether the short has occurred or not. Judging whether the short has occurred or not is performed by: summing up the A/D converted values only for the predetermined period by an integrate circuit; comparing the sum with the predetermined reference value; and outputting the signal indicating the occurrence of a short when the sum exceeds the reference value.
A second conventional method for detecting the occurrence of a short in an electric circuitis by detecting a current value at every unit time and judging whether the short has occurred or not on the basis of the variation of the absolute value of the current value. In this method, for example, when the second current value is smaller than the first current value and the third current value is bigger than the second current value, the signal indicating the occurrence of a short is produced.
However, according to the first conventional method, as only the current values for the predetermined period are summed up, it is necessary to set the reference value smaller than the summed value to detect a relatively small short current such as in case of tracking short. In case of a general short of two wires getting in touch with each other, the short current flowing between two wires is over several hundreds Ampere. In the contrary, in case of a tracking short, the short current is several Ampere to about several scores Ampere. Therefore, in this conventional method, the reference value has to be below several Ampere to about several scores Ampere to detect the tracking short. However, occasionally several Ampere to about several scores Ampere of sudden over-current could flow in an electric appliance like an incandescent lamp. It is difficult to distinguish such a sudden over-current from an electric appliance from the tracking short current. For example, in an incandescent lamp of 20 Ampere rated current, the sudden over-current could be up to 200 Ampere.
Further, according to the second conventional method, in the case of using several household electric appliances simultaneously, the variation of current value of each appliance is mixed up. When the variation of current value is mixed it could show the same amount of variation that appears in a tracking short. Therefore, it is still difficult to detect the tracking short without error from the current caused by the use of several household electric appliances.
As described above, according to the conventional methods, it is very difficult to detect a tracking short without error. It is because the current caused by a tracking short is almost the same as the current caused by the normal use of electric appliances.
It is an object of the invention to provide a method for detecting tracking short without error by distinguishing the tracking short current from the current caused by the normal use of electric appliances or electric devices.
The following comparison test was performed. The test was for comparing frequency distribution of current variation caused by a tracking short in a plug with that of a current variation caused by the normal use of several kinds of household electric appliances.
A predetermined period of time was set at about 0.2 sec. A unit of time was obtained by dividing the predetermined period into 7 parts, and a current value obtained at every unit time is the absolute value of the peak value of current in the unit time.
Whenever obtaining the current value, it was compared with a current value obtained at previous unit time and the difference between them recorded as the variation for each unit time. By repetition of the above process, a frequency distribution of variation was determined. The frequency distribution of variation was then compared to the case of a tracking short with the case of the normal use of household electric appliances.
A tracking short was intentionally formed by carbonizing the insulator between tracking electrodes in advance and supplying a AC(100V) power. Then, the waveform of current right after the occurrence of the tracking short was observed.
In addition, the waveform of the current was observed when the power was turned on with the rated voltage of several kinds of household electric appliance.
FIG. 2 and FIG. 6 resulted from the experiments showing the frequency distribution of current variation. In FIG. 2 and FIG. 6, the vertical axis indicates the variation (Ampere) at each unit time, and the horizontal axis indicates the rate (%) of the summed value of frequencies in each variation range over the total sum of frequencies in the predetermined period.
FIG. 2 shows the frequency distribution of current variation in case of the occurrence of a tracking short. FIG. 6 shows the frequency distribution of current variation for the appliance showing the biggest variation among the several kinds of household electric appliance.
In comparison of FIG. 2 with FIG. 6, FIG. 2 shows that the variation is mostly concentrated in the range of 0xcx9c4 Ampere, and FIG.6 shows that although it generally spreads broadly, the variation in the range of 5xcx9c30 Ampere is a little more than that in other ranges.
The method described herein is achieved on the basis of the result of the above test for the purpose of detecting a tracking short. The described method takes advantage of the fact that the frequency distribution of the current variation in the tracking short shows a characteristic feature.
In an embodiment, a method for detecting tracking short with the current value on the electric circuit may include: measuring the current value during the predetermined period to obtain the frequency distribution of the absolute value of current variation; comparing the rate of the frequencies in a predetermined range over the total frequency with reference value; and judging a tracking short to have occurred when said rate is above the reference value.
In some embodiments, a method may be applied to detect the occurrence of a tracking short. The method may further make it possible to detect the tracking short without error by taking advantage of the frequency distribution of current variation as a judgment condition.
In certain embodiments, a method may be provided for detecting a tracking short with the current value on an electric circuit. The method may include measuring the absolute current value at each unit time to obtain the current waveform. The current waveform may be use for judging the occurrence of the tracking short. The unit time may be obtained by dividing a predetermined period into several. The method may include calculating the variation of the current value at each unit time by getting the difference between the absolute current value at each unit time and the absolute current value of the previous or next unit time. The method may include judging a tracking short to have occurred when the frequency in the predetermined variation range for the predetermined period satisfies the pre-set reference.
In certain embodiments there may exist a plurality of variation ranges, and a reference of frequency is set for each plurality of ranges. The judgment step may judge the occurrence of a tracking short when each frequency in all the ranges satisfies the corresponding reference.
In some embodiments, a method for detecting a tracking short may include performing a judgment at every unit time during the predetermined period.
In other embodiments, the method may include: dividing the predetermined period into several unit times; performing the judgment step at every unit time; and outputting the result. In addition, the method may include erasing an oldest datum of past predetermined period at every unit time.
In other embodiments, a method may include: dividing the unit time into several; obtaining the absolute value of the peak value of the current in every said divided time; and calculating the difference between each said obtained value and previous or next value of it. The method may further include initializing and restarting the judgment step when the absolute value of the difference is below the predetermined reference value. An advantage may be providing more errorless method for detecting tracking short. More specifically, it may be possible to protect against error far better by further dividing the unit time into several parts and giving a corresponding judgment condition to each part. The reason being because it is often that the current variation exceeds the reference value only for pretty short time in case of tracking short.